1. Field of the Invention
This invention relates to distributed Bragg reflectors suitable for use in vertical cavity surface emitting lasers. More specifically, it relates to distributed Bragg reflectors that can be fabricated by etching.
2. Discussion of the Related Art
Vertical cavity surface emitting lasers (VCSELs) represent a relatively new class of semiconductor lasers. While there are many variations of VCSELs, one common characteristic is that they emit light perpendicular to a wafer's surface. Advantageously, VCSELs can be formed from a wide range of material systems to produce specific characteristics. In particular, the various material systems can be tailored to emit different wavelengths, such as 1550 nm, 1310 nm, 850 nm, 670 nm, and so on.
VCSELs include semiconductor active regions, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and contacts. Because of their complicated structure, and because of their material requirements, VCSELs are usually grown using metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).
FIG. 1 illustrates a typical VCSEL 10. As shown, an n-doped gallium arsenide (GaAS) substrate 12 has an n-type electrical contact 14. An n-doped lower mirror stack 16 (a DBR) is on the substrate 12, and an n-type graded-index lower spacer 18 is disposed over the lower mirror stack 16. An active region 20, usually having a number of quantum wells, is formed over the lower spacer 18. A p-type graded-index top spacer 22 (another confinement layer) is disposed over the active region 20, and a p-type top mirror stack 24 (another DBR) is disposed over the top spacer 22. Over the top mirror stack 24 is a p-type conduction layer 9, a p-type GaAs cap layer 8, and a p-type electrical contact 26.
Still referring to FIG. 1, the lower spacer 18 and the top spacer 22 separate the lower mirror stack 16 from the top mirror stack 24 such that an optical cavity is formed. As the optical cavity is resonate at specific wavelengths, the mirror separation is controlled so as to resonant at a predetermined wavelength (or at a multiple thereof). At least part of the top mirror stack 24 includes an insulating region 40 that provides current confinement. The protons can be implanted, for example, in accordance with the teachings of U.S. Pat. No. 5,115,442, which is incorporated by reference. The oxide layer can be formed, for example, in accordance with the teachings of U.S. Pat. No. 5,903,588, which is incorporated by reference. The insulating region 40 is usually formed either by implanting protons into the top mirror stack 24 or by providing an oxide layer. The insulating region 40 defines a conductive annular central opening 42 that forms an electrically conductive path through the insulating region 40.
In operation, an external bias causes an electrical current 21 to flow from the p-type electrical contact 26 toward the n-type electrical contact 14. The insulating region 40 and the conductive central opening 42 confine the current 21 such that it flows through the conductive central opening 42 to the active region 20. Some of the electrons in the current 21 are converted into photons in the active region 20. Those photons bounce back and forth (resonate) between the lower mirror stack 16 and the top mirror stack 24. While the lower mirror stack 16 and the top mirror stack 24 are very good reflectors, some of the photons leak out as light 23 that travels along an optical path. Still referring to FIG. 1, the light 23 passes through the p-type conduction layer 9, through the p-type GaAs cap layer 8, through an aperture 30 in the p-type electrical contact 26, and out of the surface of the vertical cavity surface emitting laser 10.
It should be understood that FIG. 1 illustrates a common VCSEL structure, and that numerous variations are possible. For example, the dopings can be changed (say, by providing a p-type substrate 12), different material systems can be used, operational details can be tuned for maximum performance, and additional structures, such as tunnel junctions, can be added.
While generally successful, VCSELs have problems. In particular, VCSEL fabrication is often difficult. For example, InP based VCSELs usually incorporate a full DBR stack comprised of InGaAsP/InP or of AlGaInAs/AlInAs as a top mirror. Such mirrors are typically 7–10 μm thick. This presents a problem when attempting proton implantation to produce current confinement because commonly available implant species can be implanted only about 4 μm when using commonly available equipment. Thus, the top DBR mirror needs to be partially etched such that the top DBR mirror has the correct dimensions for reflection, and such that implantation can be properly performed. Selective etching in InGaAsP/InP or in AlGaInAs/AlInAs material systems is difficult to do, particularly when using plasma etching. This is because the etch-rate contrast between two compositions of InGaAsP/InP or of AlGaInAs/AlInAs is not significant. It is known to use a combination of plasma and wet etching to selectively etch, but at the expense of lateral definition.
Other uses of selectively etched DBRs exist. For example, with a selectively etched DBR it would be possible to replace some of the top DBR with metal to improve heat dissipation. Furthermore, selective, controlled etching can be used to produce novel VCSELs. Therefore, a distributed Bragg reflector that can be selectively etched in a simple manner with controlled results would be beneficial. Also beneficial would be a distributed Bragg reflector that is suitable for use in vertical cavity surface emitting lasers and that can be accurately etched would be beneficial. Also beneficial would be a vertical cavity surface emitting laser having a distributed Bragg reflector with selected structures, particularly when that DBR includes an oxide structure having an aperture.